This invention relates to wireless communications systems, and more particularly, this invention relates to wireless communications systems having multi-sectored cells with multi-carrier base stations.
Cellular systems are in widespread use throughout the world and provide telecommunications to many different mobile users. Some of the cellular systems use different cells with alternate frequency bands and different carrier frequencies. A geographic area is covered with a plurality of these cells, and each includes a base station that communicates with a plurality of mobile stations. A mobile switch controller usually connects to the public switched telephone network or other communications network. Because of the increasing use of cellular telephones, it is necessary to have the base station accommodate as many different mobile units as possible. This can be accomplished by having powerful base stations, increasing the capacity to service a large number of mobile units.
Recently, it has become advantageous to divide one cell into a plurality of sectors. For example, in one common system architecture, a base station antenna has a 120xc2x0 beam width corresponding to a cell that is divided into three sectors. While a three sector division is more common, it is also possible to divide a cell into six sectors, using an antenna having a 60xc2x0 beam width, corresponding to a hex-sectored configuration.
Usually, in the preferred code division multiple access (CDMA) technology, the same frequency is used in all cells within the system architecture. When a mobile unit moves from one cell to another cell, a xe2x80x9csoftxe2x80x9d handoff occurs, where there is no switching in frequency, as compared to a xe2x80x9chardxe2x80x9d handoff where a frequency change occurs. A xe2x80x9csofterxe2x80x9d or more soft handoff occurs when a mobile unit moves from one sector to a second sector within the same cell. In this type of handoff, a mobile unit can be communicating with two or more sectors in the same cell, as compared to a xe2x80x9csoftxe2x80x9d handoff where the mobile unit communicates with two sectors in different cells. Usually, in a xe2x80x9csoftxe2x80x9d handoff, two different channel elements are allocated from two different base stations by techniques known to those skilled in the art. In a xe2x80x9csofterxe2x80x9d handoff, different channel elements are not required and the same channel element could be used for both sectors in the same cell.
As noted before, in one well known CDMA distributed base station for cellular networks, three sectors are formed in each cell, such as in Flexent(trademark), a CDMA distributed base station system manufactured and sold by Lucent Technologies Inc. This is a compact, low-cost base station. In one aspect of the Flexent(trademark) system, it operates in the 850 MHz system with the 900, 1800, dual mode 900/1800 and 1900 GSM/PCS standards. In one type of system, it can support up to 12 transceivers and works with both outdoor cellular systems, and in other types of systems, as an indoor base station. The Flexent(trademark) system, and other similar wireless systems, include a transmit (TX) radio frequency (RF) power section, as known to those skilled in the art. For example, in one type of architecture shown in FIGS. 2 and 3, a base station uses a three-sector, three-carrier system having nine ultralinear amplifier modules (ULAM), each having an associated hardware and a fixed cost. These amplifiers also consume DC electrical power and generate heat that must be dissipated. They also occupy cabinet space that could otherwise be used for additional features, and require installation and provisioning time. Thus, each ultralinear amplifier module contributes to the overall cost of the entire cellular system.
It is therefore an object of the present invention to provide a wireless communication system having a multi-carrier, multi-sector system that overcomes the disadvantages as noted above.
In accordance with the present invention, a wireless communication system has at least one cell with a plurality of sectors. A base station communicates with the mobile units contained within the cell and has a transmitter for transmitting communication signals along at least two carriers per sector within a cell. Each carrier within a sector is assigned an exclusive zone spaced from the base station from an inner zone to an outer zone. The power per carrier is reduced successively from the outer zone to the inner zone.
The base station defines a coverage area within the cell of three sectors and three carriers per sector. Each of the three carriers per sector is assigned to a respective inner zone, an intermediate zone, and an outer zone. The inner zone has handling capacity priority over the outer zone, in one aspect of the invention. The carrier of the inner zone can be activated before the carrier of the outer zone. As in many systems, the transmitter transmits the carriers as code division multiple access (CDMA) communications signals. The outer zone has an overlap of predetermined percentage, such as 15% to enhance handoff.
In yet another aspect of the present invention, the base station communicates with mobile units contained within the cell. The base station includes a transmitter for transmitting communications signals along three carriers per sector. A pair of linear amplifier modules receive and amplify the communication signals to be transmitted. Three carriers are within a sector and assigned respective inner, intermediate and outer zones spaced from the base station. The power per carrier is reduced successively from the outer to the inner zones. A three-way combiner receives each of the three carriers and a two-way divider splits the combined carriers into signals for receipt by the pair of amplifier modules.
A method of operating a base station within a cell of a wireless communication system is also set forth. Sectors are allocated within the cell. Wireless communication signals are transmitted from the base station along at least two carriers per sector. Each carrier is assigned to a different, exclusive zone spaced from the base station from an inner zone to an outer zone. The power is reduced a predetermined amount for each successive carrier from the outer to the inner zone.